A) Field of the Invention
The present invention relates to a wavelength controllable optical device, and more particularly to a wavelength controllable optical device suitable for application to a wavelength variable laser oscillator.
B) Description of the Related Art
A sampled grating laser oscillator, a vertical cavity surface emitting laser (VCSEL) integrated with a micro electro-mechanical systems (MEMS) mirror, and the like are known as wavelength variable laser oscillators.
FIG. 3A is a schematic cross sectional view of a sampled grating laser oscillator. An active layer 100 is sandwiched between a lower clad layer 101 and an upper clad layer 102. Along a light propagation direction, a first reflector region 110, a gain region 111, a phase adjusting region 112 and a second reflector region 113 are defined.
In the first and second reflector regions 110 and 113, gratings 115 and 116 are formed at the interface between the active region 100 and lower clad layer 101. The gratings 115 and 116 have the structure that a distributed Bragg reflector (DBR) is cut off at a constant pitch, and peaks of the reflection spectrum are disposed at regular intervals.
The DBR cutting off period of the grating 115 of the first reflector region 110 is different from that of the grating 116 of the second reflector region 113. The pitches of peaks of the reflection spectra of the gratings 115 and 116 are therefore different. One of a plurality of peaks of the reflection spectrum of the first reflector region 110 is superposed upon one of the peaks of the reflection spectrum of the second reflector region 113. The laser oscillator oscillates at the wavelength where the two peaks are superposed.
Carriers are injected from a common electrode 120 into the active region 100 via the lower clad layer 101. Electrodes 121, 122, 123 and 124 are formed on the surfaces of the first reflector region 110, gain region 111, phase adjusting region 112 and second reflector mirror region 113, respectively of the upper clad layer 102. As carriers are injected from the electrode 121 on the surface of the first reflector region 110 into the active layer 100, the peak position of the reflection spectrum of the first reflector region 110 is shifted. Of the peaks of the reflection spectra of the first and second reflector regions 110 and 113, peaks are superposed which are different from the peaks superposed when carriers are not injected. Therefore, the oscillation wavelength of the laser oscillator changes.
A change in the oscillation wavelength has a discontinuity corresponding to about the pitches between a plurality of peaks of the reflection spectra of the first and second reflector regions 110 and 113. As current is injected from the electrode 123 on the surface of the phase adjusting region 112 into the active layer 100, a refractive index of the phase adjusting region 112 changes so that an effective optical resonator length changes. Fine adjustment of the oscillation wavelength of the optical resonator is therefore possible.
FIG. 3B is a schematic cross sectional view of a MEMS mirror integrated VCSEL. A recess is formed in the back surface layer of a semiconductor substrate 130 and an exciting laser oscillator 139 is mounted on the bottom surface of the recess (on the upper surface of the recess as viewed in FIG. 3B). On the principal surface of the semiconductor substrate 130, an active layer 131 and a cap layer 132 are stacked in this order.
An electrode 133 is formed on the surface of the cap layer 132. An opening 133a is formed through the electrode 133 above the exciting laser oscillator 139. A mirror holder 135 is mounted on the electrode 133 via a spacer 134. The mirror holder 135 is disposed above the electrode 133 at a position spaced apart from the electrode 133 by a predetermined distance. An opening 135a is formed through the mirror holder 135 at the position corresponding to the opening 133a. A mirror 138 covers the opening 135a. The mirror 138 and exciting laser oscillator 139 constitute an optical resonator.
As a d.c. voltage is applied across the electrode 133 and mirror holder 135, the distance between the mirror holder 135 and electrode 133 is shortened by a Coulomb force. Because the mirror 138 is displaced toward the semiconductor substrate 130 side, an optical resonator length is shortened. In this manner, by changing the optical resonator length, the oscillation wavelength can be changed.
The oscillation wavelength of the sampled grating laser oscillator can be changed almost continuously by shifting the peak position of the reflection spectrum of the reflector region 110 or 113 and changing the refractive index of the phase adjusting region 112. However, adjustment becomes complicated because the refractive indices of both one reflector region 110 and the phase adjusting region 112 are to be adjusted.
The MEMS mirror integrated VCSEL utilizes a mechanical structure for displacing the mirror. A response speed is therefore low in the order of several ms.